The present invention relates to an electric power steering apparatus for vehicles which can be suitably used in passenger cars, and more particularly to a power steering apparatus of the type which, by employment of an electric motor, is capable of generating an auxiliary steering force, i.e., a force for assisting steering operations.
Hydraulic power steering apparatus are generally used as power steering apparatus which serve to lighten the operation of the steering wheel of an automobile and the like and hence to improve driving safety. However, electric power steering apparatus have recently attracted attention for the reason that they not only facilitate energy savings and increase the items which can be controlled, but also offer an advantage of making the apparatus into a smaller size which may be easily mounted in a vehicle. Such electric power steering apparatus is disclosed in, for example, the publication of Japanese Pat. Laid-Open No. 184171/1986.
The power steering apparatus described in the above publication comprises a steering shaft, an electric motor operable to generate an auxiliary steering force having a value or magnitude corresponding to the value of an electric current supplied thereto, means for detecting an operational state of the power steering apparatus, and means for controlling the value of electric current supplied to the electric motor in accordance with a value detected by the detecting means. The detecting means includes a steering torque sensor for detecting the steering torque applied to the steering shaft from a steering wheel.
It is an object of power steering apparatus to generate an auxiliary steering force, i.e., a force for assisting the operation of a steering wheel by the driver, in order to reduce the force required for the driver to turn operate the steering wheel. In the power steering apparatus described in the abovementioned publication, the value of electric current supplied to the electric motor is controlled by inputting a signal of the steering torque detected by the steering torque sensor, and the auxiliary steering force having a magnitude controlled in accordance with the magnitude of the detected steering torque is generated. However, when the magnitude of the auxiliary steering force is controlled solely on the basis of the detected steering torque, the steering force which has to be actually applied to the steering wheel by the driver remains constant regardless of the steering angle of the steering wheel because the value or magnitude of the auxiliary steering force increases and decreases according to the increase and decrease, respectively, of the value or magnitude of the detected steering torque. This hinders the driver from sensing the steering angle of wheel, and hence is unpreferable in view of the driver's feel of the steering operation.
In the aforementioned conventional power steering apparatus, the steering angle of the steering shaft connected to the steering wheel is detected by the steering angle sensor and the detected value is inputted in the controlling means so that the value of electric current supplied to the electric motor may be restricted in response to the increase in the detected value. In this way, the auxiliary steering force is reduced or eliminated and hence the steering force required for the driver to operate the steering wheel is increased when the steering angle reaches a value greater than a predetermined value.
However, the steering force actually required depends upon various factors. For example, the required steering force changes when a vehicle weight or the load applied to the wheels (front wheels) is changed. In other words, if the load on the front wheels is increased due to an increased number of passengers, an increased torque is required for "steering without driving", i.e., an increased torque is required for the steering operation effected under condition that the vehicle is not driven (the magnitude of the torque required for the "steering without driving" being proportional to the load applied to the front wheels). Also during running of the vehicle, an increased torque is required for steering operations when the load on the front wheels is increased. Since the aforementioned prior art apparatus is constructed without taking account of this fact, the auxiliary steering force becomes insufficient and hence it becomes difficult to smoothly perform steering operations particularly when the steering angle is increased.